J. AMER. SOC. HORT. SCI. 126(3):297–304. 2001.
Drought-stress Effects on Physiology, Growth and Biomass Production of Rainfed and Irrigated Bell Pepper Plants in the Mediterranean Region Sebastiano Delfine1 Dipartimento di Scienze Animali Vegetali e dell’Ambiente, Università degli Studi del Molise, via De Sanctis, 86100 Campobasso, Italy Francesco Loreto Consiglio Nazionale delle Ricerche – Istituto di Biochimica ed Ecofisiologia Vegetale, 00016 Monterotondo Scalo (Roma), Italy Arturo Alvino Dipartimento di Scienze Animali Vegetali e dell’Ambiente, Università degli Studi del Molise, via De Sanctis, 86100 Campobasso, Italy ADDITIONAL INDEX WORDS. Capsicum annuum, fluorescence, gas exchange, photosynthesis, CO2 transfer conductance, vegetative and fruit biomass, drought stress ABSTRACT. Physiological characteristics, growth, and biomass production of rainfed and irrigated bell pepper [Capsicum annuum L. var. anuum (Grossum Group) ‘Quadrato d’Asti’] plants were measured in the semiarid conditions of a Mediterranean summer to determine if drought stress effects are transient and do not affect plant growth and crop yield or are persistent and adversely affect plant growth and crop yield. A low midday leaf water potential indicated the occurrence of transient drought stress episodes in rainfed plants during the first 2 months of the study. Later on, predawn water potential also increased, indicating a persistent drought stress condition despite the occurrence of some rainfall. Photosynthesis was reduced when stress conditions developed, but the reduction was transient and limited to the central part of the day during the first 2 months. As plants aged, however, the impact of drought stress on photosynthesis was not relieved during the overnight recovery period. Stomatal conductance was reduced both during transient and permanent stress conditions while CO2 transfer conductance (i.e., conductance to CO2 inside the leaf) was only reduced when photosynthesis inhibition was unrecoverable. However, chloroplast CO2 concentration was higher in rainfed than in irrigated leaves indicating that CO2 availability was not limiting photosynthesis. Nonphotochemical quenching of fluorescence increased significantly in rainfed leaves exposed to permanent stress indicating the likely impairment of ATP synthesis. Transient inhibition of photosynthesis did not significantly affect leaf area index and biomass production, but growth was significantly reduced when photosynthesis was permanently inhibited. Fruit dry weight was even higher in rainfed plants compared to irrigated plants until drought stress and photosynthesis reduction became permanent. It is suggested that bell pepper growth without supplemental irrigation over the first part of the vegetative cycle does not impair plant growth and may even be useful to improve yield of early fruit.
Severe drought stress may impair many plant functions but the main effect is reduction of carbon fixation. This, in turn, may differentially affect plant growth and production depending on many variables such as the length of the stress, the vegetative status of the crop, and the occurrence of other environmental stress (e.g., high light irradiance and high temperatures). One of the most well known responses to drought stress is stomatal closure and the subsequent increase of resistance to CO2 diffusion in leaves (Kaiser, 1987). The concentration of CO2 at the site of its fixation (the chloroplast) may be further restricted by resistances inside the leaf mesophyll (Loreto et al., 1992). These resistances are also likely to increase in water-stressed leaves (Cornic and Massacci, 1996; Loreto et al., 1997). The view that resistances to CO2 diffusion are the only factor responsible for the reduction of carbon fixation in water-stressed leaves has often been challenged. Many studies have demonstrated that biochemical and photochemical reactions of CO2 Received for publication 23 June 2000. Accepted for publication 13 Dec. 2000. We thank Sig. Giuseppe Santarelli for measuring micrometeorological data and Sig. Luigi Santini for field farming. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 Corresponding author; e-mail:
[email protected].
assimilation may also be directly impaired by drought stress and, therefore, photosynthesis is not simply down-regulated in response to low internal CO2 (Gimenez et al., 1992; Hanson and Hitz, 1982). A recent report by Tezara et al., (1999) provided evidence that Rubisco characteristics and some photosynthesis intermediates are unaffected by mild drought stress, but ATP synthesis is sensitive to drought stress and may strongly reduce ribulose 1,5-bisphosphate (RuBP) content in water-stressed leaves. However, Tezara et al. (1999) also cautioned that this may not occur under field conditions when the stress develops slowly and plants may respond to drought by adjusting metabolism and resistances to water loss through stomata (Faver et al., 1996; Mojayad and Planchon, 1994; Ortiz-Lopez et al., 1991; Quick et al., 1992). Summer crops of the Mediterranean region generally experience recurrent drought stress episodes during their vegetative and reproductive cycles. Irrigation water is a limited resource and water-saving practices are highly encouraged. It is therefore important to determine whether drought stress causes physiological consequences in field-grown plants that are a) transient and unable to affect plant growth and crop yield, or b) persistent and limiting plant growth and crop yield. Bell pepper [Capsicum annuum var. anuum (Grossum group)]
is among the most susceptible horticultural plants to drought stress because of a wide transpiring leaf surface and high stomatal conductance (Alvino et al., 1994). An adequate water supply is required during the total growing period to obtain high yield (Doorenbos and Kassam, 1986). Under soil-water stress, pepper plants reduce leaf water potential, LAI (leaf area index) and the amount of light intercepted by the canopy (Alvino et al., 1994). Drought stress, also has been shown to affect harvested pepper fruit yield (Lurie et al., 1986), and senescence of leaves of corn (Zea mays L.) (Alvino et al., 1999) and pepper plants (Yanez et al., 1992). In the present investigation we studied physiological and metabolic processes affected by drought stress in field-grown bell pepper and quantified the effects of drought stress on physiology, growth, and productivity of pepper in environmental conditions typical of the Mediterranean region. The objectives of this study were to establish if and when water supplements are useful or necessary to support various physiological processes and avoid yield reduction of pepper and to provide guidelines for a more efficient and sustainable use of irrigation water. Materials and Methods PLANTS
MATERIAL, EXPERIMENTAL DESIGN, DROUGHT STRESS
MEASUREMENTS, AND GROWTH ANALYSIS. Plants of ‘Quadrato d’Asti’
bell pepper were transplanted on loamy soil at the six-leaf stage, in rows 0.8 m apart to obtain a crop density of 5 plants/m2. During the season, plants were regularly fertilized and weeds were controlled by hand tillage. Eight plots, each 30 m2, were formed at the beginning of June 1998 in an experimental field of Consiglio Nazionale delle Ricerche–Istituto di Biochimica ed Ecofisiologia Vegetale (Roma, Italy, 42° latitude) close to an agrometeorological field station. Plots were arranged in a randomized complete block design. Potential evapotranspiration was calculated from micrometeorological data using the Penman-Monteith formula (Doorenbos and Kassam, 1986). The values were corrected with crop coefficients depending on the crop development stage (0.4 for the initial period, 0.7 for crop development period, 1 for midseason, 0.9 for late season, and 0.85 at harvest) as suggested by Doorenbos and Kassam (1986) to provide crop evapotranspiration (ETcrop). In the control plots, ETcrop was fully restored by drip irrigation. After plant establishment, irrigation was withheld in four plots [0 days after treatment (DAT)]. Midday and the predawn leaf water potentials were used as stress indices. They were measured on 16 fully expanded leaves of different plants with a pressure chamber (model 3005; Soilmoisture Equipment Corp, Santa Barbara, Calif.). Growth was determined for seven replicates during the crop cycle. Fruit, leaves, and stems were oven dried at 75 °C for 48 h and dry weights (DWs) were recorded. Prior to drying leaf area was measured with a leaf area meter (LI-3100; LI-COR, Inc., Lincoln, Nebr). MEASUREMENTS OF PHOTOSYNTHETIC PARAMETERS. Net photosynthesis (Pn) and stomatal conductance (gs) were measured on fully expanded leaves during the crop cycle in the field with a portable gas exchange system (LI-6400) and a system described by Loreto et al. (1992), except that an infrared gas analyzer (LI6262) and a small leaf cuvette enclosing a leaf area of 4.9 cm2 were used. The leaf was irradiated by an optic fiber ring connected to a light source (KL1500; Schott, Mainz, Germany). Measurements were recorded on seven fully expanded leaves selected
randomly on different plants. Simultaneous gas exchange and fluorescence measurements were made using a variable irradiance when measuring the light response of photosynthesis, and at a saturating irradiance (1800 µmol·m–2·s–1) for other measurements. In all cases the leaf temperature was maintained at 25 °C by circulating thermostatically regulated water in the body of the cuvette. Photosynthetic O2 sensitivity was calculated from the relation: (Pn2 – Pn20)/Pn2, where assimilation Pn2 and Pn20 are photosynthesis at low (2 kPa) and ambient (20 kPa) O2 pressure. Fluorescence was monitored as described by Loreto et al. (1992), except that the terminal end of the optic fiber used for fluorescence measurements and for saturating light pulses (10,000 µmol·m–2·s–1) was inserted inside the fiber ring normal to the leaf plane. This allowed the fluorescence fiber to reach the cuvette without shading the leaf. The ratio between variable and maximal fluorescence (Fv/Fm) was measured on dark-adapted leaves for 30 min to estimate the effect of drought stress on photochemical efficiency. The fluorescence emission in response to actinic and saturating light was measured to estimate the rate of electron transport (Jf) according to the following equation: Jf = DF/Fm × a × 0.5 × photosynthetically active radiation (PAR), where DF/Fm is quantum yield of electron flow to photosystem II (Genty et al., 1989). The overall leaf absorptance (a) was measured with an integrating sphere (LI1800-12S) throughout the experiment. The factor 0.5 was chosen assuming that light is distributed equally between the two photosystems (Loreto et al. 1992). PAR is the incident irradiance. Nonphotochemical quenching of fluorescence (qNP) was estimated according to van Kooten and Snel (1990) from maximal and minimal fluorescence in the dark (Fm and Fo) and at an irradiance of 800 µmol·m–2·s–1 (F’m and F’o): qNP = 1 – (F’m – F’o)/ (Fm – Fo). Transfer conductance (gw) of CO2 was determined as described by Loreto et al. (1994). Briefly, the electron transport rate calculated by gas exchange (Jc) and measured by fluorescence (Jf) are compared at an irradiance of 800 µmol·m–2·s–1. Measurements at higher irradiance would be imprecise because of the low fluorescence yield (Loreto et al., 1994). If Jf = Jc under nonphotorespiratory conditions (2 kPa O2) but Jf > Jc under photorespiratory (ambient air) conditions, it is assumed that ci [the intercellular CO2 partial pressure used to calculate Jc according to von Caemmerer and Farquhar, (1981)] is higher than cc (the CO2 partial pressure in the chloroplasts). The internal resistance which causes the decrease from ci to cc can be calculated by assuming that Jf is the actual electron transport rate. Substituting Jc with Jf: gw = Pn/[(ci – Γ* × (Jf + 8 × (A + Rd)]/Jf – 4 × (A + Rd), where Pn is the photosynthetic rate, Rd is dark respiration as measured from gas exchange on the dark adapted leaf, and Γ* is the CO2 compensation point that would occur in the absence of Rd, calculated according to Brooks and Farquhar (1985). The fluorescence gas exchange method has been tested under a wide range of conditions and on many plant species. Detailed descriptions of the method, its sources of error, and comparisons with measurements of gw determined by on-line fractionation of stable carbon isotopes can be found in previous reports (Harley et al., 1992; Loreto et al., 1994). Transfer conductance measurements of CO2 were repeated on seven different leaves of different plants. RUBISCO ANALYSIS. Leaf samples (3.8 cm2) were frozen in liquid nitrogen immediately after gas-exchange measurements to measure Rubisco activity and content. Briefly, frozen leaves were ground in a chilled mortar with 30 mg polyvinylpolypyrrolidone
Plants were cultivated in environmental conditions typical of summer in central Italy. Days were cloudless over most of the experimental period. The maximum temperature was generally >30 °C for 2 months after starting the treatment and the minimum temperature was always >10° C (Fig. 1). The seasonal evapotranspiration demand (Fig. 2) was not met by precipitation which was scant particularly during the first month of the experiment (Fig. 1). Photosynthesis and stomatal conductance under saturating irradiance (1800 µmol·m–2·s–1) and optimal leaf temperature (25
°C) were similar for the first 2 months in irrigated and rainfed plants (Fig. 3). Transient and significant reductions of these parameters were observed in rainfed leaves 22 to 23 DAT and 42 DAT, just before two rainfalls (Fig. 1). After ≈2 months, Pn and gs of rainfed plants became significantly lower than in irrigated plants and this difference was observed until the end of the experiment (Fig. 3). Three sets of field measurements representative of conditions when Pn and gs were not affected (14 DAT), transiently affected (42 DAT) or permanently affected (66 DAT) by drought stress are presented in Figs. 4 to 6. Predawn and noon water potentials were not different in irrigated and rainfed plants 14 DAT (Fig. 4A and B). However, noon water potential of rainfed plants was significantly lower (more negative) than in controls 42 DAT. At 66 DAT this difference was also evident in the predawn water potential. Light response of Pn was similar in irrigated and rainfed plants 14 DAT (Fig 5A). At 42 DAT the rainfed leaves showed significantly lower Pn with respect to irrigated leaves at irradiances >500 µmol·m–2·s–1 (Fig. 5B). Finally, at 66 DAT inhibition of Pn in rainfed leaves was evident at irradiances >200 µmol·m–2·s–1 (Fig. 5C). In all cases there were no differences between irrigated and rainfed leaves in the (linear) response of Pn at irradiances 200 µmol·m–2·s–1 and saturated at 600 µmol·m–2·s–1 (≈1/3 of the maximum solar irradiance) in rainfed leaves but the linear response of photosynthesis to light (i.e., the response at irradiances
